Electron Acceptors and Electron Tower Theory

In situ bioremediation can be divided into natural and enhanced methods. As noted in the previous section, naturally occurring bioremediation occurs when a sufficient energy source, carbon source, electron acceptor concentration, and nutrient concentration are available to a native biological population. The rate of naturally occurring bioremediation of BTEX compounds is often limited by either the concentration of an appropriate electron acceptor or a nutrient needed during the transformation of the BTEX into a non-toxic compound. Enhanced in situ bioremediation attempts to stimulate biodegradation by adding either the limiting electron acceptor or the appropriate nutrients to the subsurface environment until the (hydro)carbon substrate becomes the limiting factor in reaction kinetics.

Typical electron acceptors utilized by microorganisms are oxygen, nitrate, iron (III), sulfate, and carbon dioxide. When oxygen is utilized as the electron acceptor, microbial respiration is termed aerobic. When other electron acceptors are utilized, it is termed anaerobic. Depending on the mode of respiration, microbes can be classified into three categories: (1) aerobic; (2) anaerobic; and (3) facultative Aerobes thrive only in oxygenated environments using dissolved oxygen as an electron acceptor. Strict anaerobes grow only under highly reduced conditions, where oxygen is effectively absent. Strict anaerobes use electron acceptors such as sulfate or carbon dioxide. Many microorganisms are able to adapt to both aerobic and anaerobic conditions, but are typically more active in the presence of oxygen .These organisms are termed facultative, and most microbes utilizing nitrate as an electron acceptor tend to be facultative.The following figure adapted from Jorgensen (1989) illustrates the sequence and products of electron acceptor utilization for oxidation of organic carbon.

The electron tower, as depicted above, relates the amount of energy a given microbial population can gain from electron acceptor to the electron acceptors position on the 'tower'. Microbes tend to oxidize organic substrates by the using the electron acceptor that provides the most energy. Note that oxygen, which is at the top of the electron 'tower', provides microbes with more free energy (via oxygen reduction) than any other electron acceptor. Carbon dioxide, which is used as the electron acceptor by methanogenic bacteria, yields the least energy of all the electron acceptors, and is therefore located at the bottom of the 'tower'. Thus, the electron tower provide above schematically depicts the order of electron acceptor utilization based on the free energy a given microbial population can gain from reduction of a given electron acceptor.

Field evidence also seems to suggest that natural in situ bioremediation may employ different electron acceptors at various locations throughout a given site. Lyngkilde et al. (1991) report an electron acceptor utilization order determined by measuring field concentrations for each electron acceptor. The trends observed at this observed at this site, as schematically depicted in the figure below, indicate that respiratory conditions of the plume vary from highly reactive aerobic conditions, through anoxic nitrate and iron reduction, to highly reduced sulfate and methanogenic conditions. Note that the order of electron acceptor respiration agrees well with the utilization order predicted by the electron tower theory.

Figure adapted from Lyngkilde et al., 1991. 

Source: Brauner (1995)